For at least the threats that can be seen.

“The government will soon be sending warnings of national emergencies on wireless phones, Web sites and hand-held computers. The new digital system will update the emergency alerts planned — but never used — during the Cold War in the event of a nuclear strike. More likely, these 21st-century technologies will carry warnings of natural disasters and terrorist attacks. The Homeland Security Department, through the Federal Emergency Management Agency, expects to have the system working by the end of next year.”

http://www.cnn.com/2006/TECH/ptech/07/12/alert.system.ap/index.html